I am using ubuntu for 4 months. I am starting to notice that, my CPU usage is becoming 100% very frequently and my system becomes unresponsive. I don't run any CPU intensive tasks, just nautilus, transmission, VLC, pidgin, firefox and chrome. I suspect, Chrome and firefox are responsible for CPU hogging. What can I do?

I second the flash suspect. I find that sometimes it gives me problems like that. Also Gnome-DO if you have it installed will sometimes freak out and use your entire CPU (which is why I use Synapse now).
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zookaliciousApr 20 '11 at 18:47

Flash is notorious, especially if you use sites like Super Poke Pets which open multiple flash sessions at once. [I actually saw 80 network connections between my wife's browser and the SPP site and when she was using 1 instance of firefox on 1 SPP page with multiple habitats.]
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jwernernyApr 20 '11 at 19:53

Indeed, when you run so many processes at once, even if alone they aren't resource intensive, they all take up resources which contributes to the 100% CPU usage.
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Thomas W.Apr 20 '11 at 17:17

I don't agree that the OP is running a lot of processes. Nautilus, Transmission, VLC, Pidgin, Firefox, and Chrome don't represent a heavy load. It is true, though, that playing videos takes a bit of CPU, especially if you're also running Compiz. The most likely suspect is some website(s) running in one of your browsers.
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Scott SeveranceApr 21 '11 at 5:31